The present invention relates to accurate radiation dosimetry in personnel and environmental monitoring. More particularly, the present invention relates to a CaSO4 based thermoluminescent (TL) detector bonded with a phosphorous compound and a method for fabricating thereof.
Typically represented by a thermoluminescent dosimeter (TLD) and a film badge, personnel dosimeters are useful for measuring of radiation dose equivalent to the human body.
When TLD is irradiated with radiation (e.g., X-ray, xcex3 ray, xcex2 ray, etc.) and then heated, electrons captured in traps are excited to the conduction band. In the conduction band, the electrons migrate and recombine with trapped holes which are recombination centers, thereby releasing the energy as thermoluminescence, in which the amount of the luminescence is proportional to the radiation dose irradiated to TL material. The TLD can be measured very accurately over a broad range of 10xe2x88x926xcx9c103 Gy, and pelletized to any size and form as desired. Therefore, the TLD is extensively used for radiation monitoring, finding numerous applications in personnel and environmental monitoring, radiotherapy and urgent environmental radioactive management [M. Oberhofer and A. Scharmann, Applied Thermoluminescence Dosimetry, Adam Hilger Ltd., Bristol 1981; R. Chen and Y. Kirsh, Analysis of Thermally Stimulated Processes, Pregamon Press, Oxford, 1981].
Meanwhile, the film badge is based on photosensitization of film by radiation. The radiation amount absorbed is determined by developing and measuring the darkness of the sensitized film. The film badge has the advantage of permanently preserving record of measuring the exposed radiation dose, but suffers from the disadvantage of substantial fading due to temperature and humidity fluctuations of the environment and being poor in radiation sensitivity. For these reasons, film badges are now replaced with TLD badges. Active research has been directed to the applications of TLD and the development of novel TL materials of high TL sensitivity in United States, United Kingdom and Japan [R. M. Hall and C. N. Wright, Health Phys., Pergamon Press, 1968, 14, 37-40; G. Cai, K. Geng, Q. Wang, Radiat. Prot. Dosim., 1995, 60, 259-262].
TLD materials are -classified into two categories: tissue equivalent TL material, such as LiF, Li2B4O7 and MgB4O7, having effective atomic numbers similar to air (Zeff=7.64) or human tissue (Zeff=7.42); and non-tissue equivalent TL material (CaSO4, and CaF2) similar in atomic number to bone (Zeff=14). Interacting with photons similar to human tissue the tissue equivalent TL material is favorable in measuring personnel dose equivalent. However, it has the disadvantage of lower TL sensitivity, compared with the non-tissue equivalent TL material. On the other hand, the non-tissue equivalent TLD is of such high TL sensitivity as to measure very low dose, such as environmental radiation. However, the non-tissue equivalent TLD for use in measurement of personnel dose equivalent requires an additional filter for energy dependence compensation because of high-energy response to photons.
ICRP 60 recommendations [ICRP, 1990 Recommendations of the International Commission on Radiological Protection, ICRP Publication 60, Pergamon Press, Oxford, N.Y., 1990] require that the dose is maintained as low as reasonably achievable (ALARA) for limiting stochastic effects based on the hypothesis of linear non-threshold. In order to maintain the ALARA dose, more accurate dose measurements should be conducted in the low dose range of 10xe2x88x927 to 10xe2x88x924 Gy. To this end, higher sensitivity TL materials or higher sensible dosimetric systems are needed. Suitable for this purpose is the CaSO4:Dy TL material. However, the detectors cannot be manufactured by use of CaSO4:Dy TL powders alone. For the preparation of detectors, the powders are contained in capsules or specific containers, or mixed with Teflon. The CaSO4:Dy TL detector mixed with Teflon does not show high TL sensitivity which is the major advantage of CaSO4:Dy powders, because of low content (15xcx9c30 wt %) of CaSO4:Dy powders on the basis of the total weight of the detector.
For wider applications of powdered CaSO4:Dy TL materials, there is a widely recognized need for a solidified CaSO4:Dy TL detector. Research for pelletizing CaSO4:Dy TL materials has been continuously carried out [D. R. Vij, Thermoluminescent Materials, 142-179, PTR Prentice-Hall, New Jersey, 1993; G. A. M. Webb J. E. Dauch and G. Bodin., Operational evaluations of a new high intensity thermoluminescent dosimeter, Health Phys., 1972, 23, 89-94; A. M. P. L. Fordon and R. Muccillo, Thermal neutron detection by activation of CaSO4:Dy+KBr thermoluminescent phosphor, Int. J. Appl. Radiat. Isot., 1979, 30, 571-573; S. P. Morata, A. M. P. Gordon, E. N. D. Santos, L. Gomes, L. L. Compos, L. Prado, M. M. F. Vieira and V. N. Bapat, Development of a state dosimeter based on thermoluminescent CaSO4:Dy crystals Nucl. Istrum. Methods, 1982, 200, 449-455; M. Prokic, Improvement of the thermoluminescence properties of the non-commercial dosimetry phosphors CaSO4:Dy and CaSO4:Tm, Nucl. Instrum. Methods, 1978, 151, 603-608; S. S. Shastry, S. S. Shinde and R. C. Bhatt, Thermoluminescence response of CaSO4:Dy sintered pellets, Int. J. Radiat. Isot. 31. 1980, 4, 244-245; M. Prokic, Thermbluminescent characteristics of calcium sulphate solid detectors, Radiat. Prot. Dosim., 1991, 37, 271-274]. Generally, pelletizing TL materials can be achieved with TL powders alone or in combination with a suitable binder. When pure CaSO4:Dy powders are molded under pressure and then sintered, the resulting pellet is difficult to use as a detector because of its low bonding strength. In the case of pure CaSO4:Dy TL materials, however, no better alternatives have been developed yet. Hence, Teflon is widely used for pelletizing CaSO4:Dy TL powders [G. A. M. Webb, J. E. Dauch and G. Bodin. Operational evaluations of a new high intensity thermoluminescent dosimeter, Health Phys., 1972, 23, 89-94]. The CaSO4:Dy detector using Teflon as a binder has such a Teflon content of as high as 70-85 wt % that its TL sensitivity is poor as mentioned above. What is worse, Teflon is vulnerable to heat.
In addition, use of KBr, NaCl and Mg3(BO3)2 as binders in preparing CaSO4:Dy-based TLD has been studied [A. M. P. L. Fordon and R. Muccillo, Thermal neutron detection by activation of CaSO4:Dy+KBr thermoluminescent phosphor, Int. J. Appl. Radiat. Isot. 1979, 30, 571-573; S. P. Morata, A. M. P. Gordon, E. N. D. Santos, L. Gomes, L. L. Compos, L. Prado, M. M. F. Vieira and V. N. Bapat, Development of a state dosimeter based on thermoluminescent CaSO4:Dy crystals Nucl. Istrum. Methods 1982, 200, 449-455; M. Prokic, Improvement of the thermoluminescence properties of the non-commercial dosimetry phosphors CaSO4:Dy and CaSO4:Tm, Nucl. Instrum. Methods, 1978, 151, 603-608; S. S. Shastry, S. S. Shinde and R. C. Bhatt, Thermoluminescence response of CaSO4:Dy sintered pellets, Int. J. Radiat. Isot. 1980, 31. 4, 244-245]. However, none of the detectors using the above binders are higher in TL sensitivity, compared to Teflon-embedded detectors. Another developed example is a CaSO4:Dy TL detector using multi component inorganic binding substance in a small quantity [M. Prokic, Thermoluminescent characteristics of calcium sulphate solid detectors, Radiat. Prot. Dosim., 1991, 37, 271-274]. Also in South Korea, CaSO4:Dy materials are under study and a CaSO4:Dy TL detector mixed with Teflon was developed [Choi Tae-Jin, Kim Do-Sung, Do Shi-Hong, La Byung-Ook, Kang Young-Ho, CaSO4:Dy thermoluminescent dosimeter preparation and physical properties, New Physics 1986, 26(6) 506-512].
Aiming to solve the above problems, much effort was made by the present inventors to develop a novel TL detector.
It is found that a phosphorous precursor can be converted into a suitable binder when being molded under pressure and sintered, together with CaSO4 based TL powders, and the resulting TL detector can be manufactured in various forms in addition to exhibiting high TL sensitivity and excellent mechanical properties.
Accordingly, it is an object of the present invention to provide solidified TL detectors of high TL sensitivity.
It is another object of the present invention to provide a method for fabricating novel CaSO4 based TL radiation detectors with a phosphorous compound acting as a binder.
In accordance with an embodiment of the present invention, there is provided a CaSO4 based TL detector, containing a phosphorous compound as a binder.
In accordance with another embodiment of the present invention, there is provided a method for fabricating a CaSO4 based TL detector, comprising the steps of: mixing CaSO4 based TL powder with a precursor of phosphorous compound; molding the mixture powder under pressure; and sintering the molded body.